The ability to selectively target chemotherapy has immense value in clinical practice. Cancer is a leading cause of death in the developed world, with one in every three people developing cancer during his or her lifetime. There are many treatment options for cancer including surgery, chemotherapy, radiation therapy, immunotherapy, and monoclonal antibody treatment. Unfortunately, for many patients cancer treatment options are limited and response rates remain low.
Surgery is the oldest effective form of tumor therapy and can often result in a complete cure, depending of the type and nature of the tumor. Many tumors, however, occur in locations and/or number that make surgery impossible or impractical. Also, surgical debulking is not guaranteed to remove all abnormal cells, particularly in the case of tumors located in the brain where maximum preservation of normal tissue is desired. Residual abnormal cells pose an increased risk of tumor re-growth and/or metastasis.
Radiation therapy is often used as an adjunct to surgery. Various types of radiation, both from external and implanted sources, have been used with some success. Low linear-energy-transfer (LET) sources, such as β-particles and γ-rays, require repeated treatments over extended periods of time to produce any significant reduction in tumor cells. High LET sources, such as neutrons, protons or α-particles, do not require oxygen to enhance their biological effectiveness. External beam therapy has been available for decades, however, significant radiation damage occurs to normal tissues, and patients often succumb to widespread radiation-induced necrosis (Laramore, et al., Cancer, 1978, 42(1), 96-103).
Chemotherapy is used in attempts to cure or palliate cancer. Small molecule chemotherapeutics target rapidly dividing cells, halting cell proliferation by interfering with DNA replication, cytoskeletal rearrangements and/or signaling pathways that promote cell growth. Disruption of cell division slows the growth of malignant cells and may also kill tumor cells by triggering apoptosis. Alkylating agents, such as bis(2-chloroethyl)amine derivatives, act by covalent interaction with nucleophilic heteroatoms in DNA or proteins. It is believed that these difunctional agents are able to crosslink a DNA chain within a double helix in an intrastrand or interstrand fashion, or to crosslink between DNA, proteins or other vital macromolecules. The crosslinking results in inhibitory effects on DNA replication and transcription with subsequent cell death. Since these drugs also indiscriminately kill normal populations of rapidly proliferating cells, such as those found in the immune system and in the gastrointestinal tract, side effects that limit tolerated doses, are common.
The harsh side effects and the ultimate failure of most chemotherapy regimens have motivated investigation of alternatives, including drugs that target specifically tumor cells. Normal cells and tumor cells differ markedly in nutrient and energy metabolism, a phenomenon known as the Warburg effect (Ganapathy, et al., Pharmacol Ther, 2009, 121(1), 29-40; and Vander Heiden, et al., Science, 2009, 324(5930), 1029-1033). Enhanced proliferation in tumor cells places increased demand for nutrients to serve as building blocks for the biosynthesis of macromolecules and as sources of energy. Tumor-selective nutrient accumulation is most clearly evident in imaging studies of human tumors using positron emission tomography (PET) and [18F]-fluorodeoxyglucose (FDG). FDG accumulates at high levels in many kinds of solid tumors and is thought to be taken up into tumor cells by sugar transporters. Amino acids are the primary source of cellular nitrogen, used for nucleotide, glutathione, amino sugar, and protein synthesis. In addition, tumors often utilize the carbon skeletons of amino acids as an oxidative fuel source for ATP generation in addition to glucose and fatty acids (Baggetto and Biochimie, 1992, 74(11), 959-974; Mazurek and Eigenbrodt, 2003, Anticancer Res, 2003, 23(2A), 1149-1154; and DeBerardinis, et al., Proc Natl Acad Sci USA, 2007, 104(49), 19345-19350). Therefore, tumor cells must express select specific transporters to satisfy maintenance and growth requirements for nutritional amino acids. To compete with surrounding tissue for nutrients, tumor cells up-regulate levels of certain transporters to allow for more efficient extraction of nutrients than that of the host tissue.
Amino acid transport across the plasma membrane in mammalian cells is mediated by different transport “systems” such as the sodium-dependent systems A, ASC and N, and sodium-independent system L (Christensen, Phys Rev, 1990, 70, 43-77). System L is a ubiquitous plasma membrane amino acid transport system that is characterized by the sodium-independent uptake of bulky, hydrophobic amino acids and its high affinity interaction with 2-amino-bicyclo[2,2,1]heptane-2-carboxylic acid (BCH). System L activity is presently attributed to four sodium-independent transporters (LAT1-4). However, most cancers over-express only one member, the large amino acid transporter 1 (LAT1/4F2hc). This transporter is a heterodimer consisting of a light chain (LAT1) that constitutes the transporter and a heavy chain 4F2hc (also known as CD98, or Tumor Antigene TA1) that is required for proper targeting of the light chain to the plasma membrane. The expression and activity of LAT1/4F2hc correlates with cell proliferation and cancer growth; and up-regulation of LAT1/4F2hc has been observed, for example, in cancers of brain, colon, lung, liver, pancreas, and skin (Jager, et al., J Nucl Med, 1998, 39(10), 1736-1743; Ohkame, et al., J Surg Oncol, 2001, 78(4), 265-267; Tamai, et al., Cancer Detect Prev, 2001, 25(5), 439-445; Kim, et al., Anticancer Res, 2004, 24(3a), 1671-1675; Kobayashi, et al., Neurosurgery, 2008, 62(2), 493-503; Imai, et al., Histopathology, 2009, 54(7), 804-813; and Kaira, et al., 2009, Lung Cancer, 66(1), 120-126). Furthermore, the expression of LAT1/4F2hc has been used as an independent factor to predict poor prognoses in patients with astrocytic brain tumors, lung cancer, and prostate cancer (Nawashiro, et al., Int J Canc, 2006, 119(3), 484-492; Kaira, et al., Lung Cancer, 2009, 66(1), 120-126; Kaira, et al., Cancer Sci, 2008, 99(12), 2380-2386; and Sakata, et al., Pathol Int, 2009, 59(1), 7-18). Inhibition of LAT1/4F2hc-mediated transport with non-metabolizable amino acids such as BCH can reduce growth and induce apoptosis in cancer cells in vitro (Kim, et al., Biol Pharm Bull, 2008, 31(6), 1096-1100; Shennan and Thomson, Oncol Rep, 2008, 20(4), 885-889; and Kaji, et al., Int J Gynecol Cancer, 2010, 20(3), 329-336). Clinical studies have shown that the specificity and positive predictive value of L-[3-18F]-α-methyltyrosine ([18F]-FAMT) PET is superior to [18F]-FDG PET. The uptake of [18F]-FAMT in tumors has been closely correlated with LAT1 expression (Haase, et al., J Nucl Med, 2007, 48(12), 2063-2071; Kaira, et al., Clin Cancer Res, 2007, 13(21), 6369-6378; and Urakami, et al., Nucl Med Biol, 2009, 36(3), 295-303).
In particular, melphalan is an effective chemotherapy drug used in treating multiple myeloma, ovarian cancer, retinoblastoma, and other hematopoietic tumors. However, substrates such as gabapentin are reported to be transported much more rapidly than melphalan (Uchino, et al., Mol Pharmacol 2002, 61(4), 729-737). It is widely believed that uptake of melphalan (Alkeran®, otherwise known as L-Phenylalanine Mustard, or L-PAM) into cells is mediated by amino acid transporters. Melphalan is an alkylating agent linked to the essential amino acid phenylalanine. Because normal cells and tumor cells differ markedly in nutrient and energy metabolism (Warburg effect) (Vander Heiden, et al., Science, 2009, 324(5930), 1029-1033), melphalan was introduced into clinical practice with the expectation that it would preferentially accumulate in rapidly dividing tumor cells compared to normal cells, thereby increasing its overall therapeutic index. Surprisingly, melphalan caused many of the same side effects as other conventional alkylation agents, including myelosuppression. In a series of publications, Vistica et al. examined melphalan transport in different cell types and identified two independent transport systems for melphalan. One system, presumed to be System L, is characterized by the sodium-independent uptake of bulky, hydrophobic amino acids and its sensitivity toward inhibition with 2-amino-bicyclo[2,2,1]heptane-2-carboxylic acid (BCH) (Vistica, Biochim Biophys Acta, 1979, 550(2), 309-317). A second transport system is sodium-dependent, exhibits its highest affinity for leucine, but is insensitive to both BCH and the system A-specific inhibitor α-amino-isobutyric acid (AlB) (Vistica, Biochim Biophys Acta, 1979, 550(2), 309-317). Although LAT1 is overexpressed on the cell surface of almost all tumor cells regardless of the tissue of origin, response rates to melphalan are low for most cancer types, and the drug is only approved for the treatment of multiple myeloma and ovarian cancer. Melphalan is a poor substrate for LAT1 compared to other large amino acids such as phenylalanine and leucine (Uchino, et al., Mol Pharmacol 2002, 61(4), 729-737; and Hosoya, et al., Biol Pharm Bull, 2008, 31(11), 2126-2130). Nitrogen mustard derivatives with higher selectivity toward the LAT1/4F2hc system could reduce side effects associated with nitrogen mustard therapy, allow for an increase in dose, and extend the use into other areas of cancer treatment.
Although the potential for active transport strategies for increasing drug uptake into tumor cells is known and generally accepted, chemotherapeutics and tumor imaging agents have in general not been optimized for transporters known to be over-expressed in tumor cells. While the general concept of using LAT1/2Fhc-selective compounds to deliver therapeutic agents to tumors is appreciated, the existing art gives no guidance as to how one prepares a composition that exploits LAT1/4F2hc selective compounds. Thus, there is a need for new therapeutic agents that are more selective toward LAT1/4F2hc.
Several amino acid-related drugs that are substrates of the LAT1/4F2hc transporter are known, including L-Dopa, 3-O-methyldopa, droxidopa, carbidopa, 3,3′,5′-triiodothyronine, thyroxine, gabapentin, and melphalan (Uchino, et al., Mol Pharm 2002, 61(4), 729-737; and del Amo et al., Eur J Pharm Sci, 2008, 35(3), 161-174).